1. Technical Field
The present invention relates to a line head and an image forming apparatus using the same and in particular, to a line head that forms a row of imaging spots by projecting a light-emitting element row onto an irradiated surface using a microlens array and an image forming apparatus using the same.
2. Related Art
JP-A-2-4546 proposes an optical writing line head, in which a plurality of LED array chips are disposed in the LED array direction and the LED array of each LED array chip is projected onto a photoconductor in an enlarged manner by a positive lens disposed corresponding thereto such that images of light-emitting dots at the ends of the LED array chips adjacent on the photoconductor are formed adjacent to each other with the same pitch as a pitch between images of light-emitting dots of the same LED array chips, and an optical reading line head in which the optical path is oppositely set.
Furthermore, JP-A-6-344596 proposes making the focal depth large by configuring a positive lens with two lenses in the same arrangement as disclosed in JP-A-2-4546 and making projected beams approximately parallel beams.
Furthermore, JP-A-6-278314 proposes an optical writing line head in which LED array chips are disposed in two rows at distances there between, the repeat phase is shifted by a half period, and positive lens arrays are disposed in two rows in a condition where each positive lens corresponds to each LED array chip such that images of the light emitting dot array on a photoconductor are aligned.
In the above known techniques, if an angle of view of each positive lens increases, a decrease in the amount of peripheral light becomes large according to a fourth power of cosine rule (shading). In order to prevent the concentration unevenness of a printing image caused by the shading, it is necessary to make constant the light amount of each pixel (light-emitting dot image) on the image surface. In order to do so, however, the shading should be corrected by changing the light amount of the light source (light-emitting dot) for every light-emitting dot. However, the emission intensity of a light source pixel (light-emitting dot) affects a life characteristic. Accordingly, if the shading of an optical system becomes large, the light amount unevenness of the light-emitting dot pitch occur with time even if the uniform image surface light amount is obtained in an early stage by adjusting the light amount for every light-emitting dot. This causes the image concentration unevenness.
In addition, in case of arranging a plurality of rows of light-emitting dot arrays in the sub-scanning direction, the width of an optical writing line head in the sub-scanning direction becomes large if the lens diameter of an optical system increases. As a result, it becomes difficult to make an image forming apparatus small.